Airborne Locator Of An Underground Object

ABSTRACT

A system for tracking a below-ground transmitter from an aerial receiver. The receiver has an antenna assembly, a processor, and a propulsion system. The antenna assembly detects the magnetic field from an underground transmitter and generates an antenna signal. The processor is programmed to receive the antenna signal and generate a command signal, which moves the receiver to a position above the transmitter. Once in the desired position, which may be a reference plane at a fixed elevation, the antenna assembly measures the magnetic field to determine the location of the drill bit along borepath.

FIELD

The present invention relates generally to the locating of drill bitposition during horizontal directional drilling and other excavationoperations and specifically to the use of a drone to track progress of aborehole.

SUMMARY

The invention is directed to a system for tracking a drill bit. Thesystem comprises a drill rig, a drill string, a downhole tool, a drillbit, a dipole magnetic field transmitter, and a self-propelledautonomous receiver. The drill string has a first end and a second end.The first end is operatively connected to the drill rig. The downholetool is connected to the second end of the drill string. The dipolemagnetic field transmitter is supported by the downhole tool. A dipolemagnetic field is emitted from the downhole tool at an undergroundlocation. The drill bit is connected to the downhole tool. The receivercomprises an antenna and a processor. The antenna detects the dipolemagnetic field. The processor is configured to perform a method. Themethod comprises the steps of maintaining the autonomous receiver in areference plane above the ground, receiving a signal indicative of thefield detected by the antenna, determining a direction of a null withinthe dipole magnetic field, and directing the autonomous receiver to movealong the reference plane to the null point.

In another embodiment, the invention is directed to a method. The methodcomprises transmitting a dipole magnetic field from a transmitter at anunderground location and engaging a propulsion system to lift anautonomous receiver into the air to a predetermined reference elevation.The method also comprises detecting the dipole magnetic field at thereference elevation using an antenna assembly disposed on the autonomousreceiver, and moving the receiver with the propulsion system to aposition above the transmitter and at a front null point of the magneticfield using the detected magnetic field, while keeping the receiver atthe predetermined reference elevation. Thereafter, the signal strengthof the magnetic field and the orientation of the magnetic field in threedimensions is measured using the antenna assembly with the receiver atthe null point to determine a vertical distance between the transmitterand the receiver. The altitude of the receiver above a ground surface isdetected. Thereafter, an actual depth of the underground location isdetermined using the altitude and the vertical distance.

In yet another embodiment, the invention is directed to a systemcomprising a signal transmitter and a self-propelled autonomousreceiver. The transmitter is disposed at an underground location andgenerates a dipole magnetic field from the underground location. Thereceiver comprises an antenna assembly, a processor, and a propulsionsystem. The antenna assembly detects the magnetic field and generates anantenna signal. The processor receives the antenna signal and generatesa command signal. The propulsion system receives the command signal andmoves the receiver to a position above the transmitter and at a frontnull point of the magnetic field using the detected magnetic field whilekeeping the receiver at a predetermined reference elevation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall plan view of a horizontal directional drillingoperation using a tracker of the present invention.

FIG. 2 is a diagrammatic representation of the tracker of FIG. 1.

FIG. 3 is a block diagram of a tracker constructed to detect and processmagnetic field signals from a transmitter.

FIG. 4 is an illustration of flux lines radiating from a transmitter, asdepicted in the x-y plane.

FIG. 5 is a plan view of a locator disposed above an underground utilityemitting a magnetic field.

FIG. 6 is a diagrammatic representation of the locator of FIG. 5.

FIG. 7 is an overhead plan view of an underground utility.

FIG. 8 is an overall plan view of a horizontal directional drillingoperation using a tracker, showing a ground surface with a variablealtitude and a tracker having a constant elevation.

FIG. 9 is a side plan view of an underground utility, wherein the groundsurface has a variable elevation and the tracker maintains a constantelevation.

DETAILED DESCRIPTION

The horizontal directional drilling (HDD) industry traditionally useswalk-over tracking techniques to follow the progress of a bore, to findthe surface location immediately above the drill bit, and to determinethe depth of the drill bit from that surface location. The primarytracking tools are a subsurface transmitter and a hand-carried surfacereceiver. The transmitter, located in or very near a cutting tool,generally emits a magnetic dipole field created by a single coil dipoleantenna. The transmitted dipole field can be used for both location andcommunication with the above ground receiver. Hand-held receivers arevery useful and are appropriate in most drilling operations because theoperator can walk along the borepath to track the cutting tool. However,from time-to-time obstructions or restrictions may prevent an operatorfrom walking along the entire borepath. Thus, there remains a need forreceivers that are capable of locating a cutting tool when the operatoris not able to position himself and the receiver over the cutting tool.

In the same way, locating an existing underground utility is a criticalpart of preparing to bore in the subsurface. Thus, a receiver may beprovided with a locating antenna on board to detect a field generatedabout an underground utility, such as a wireline or pipeline. Mappingthe depth and path of such underground utilities, without regard toobstructions located on the surface, is advantageous.

With reference now to the drawings in general and FIG. 1 in particular,there is shown therein an “HDD” system 10 for use with the presentinvention. FIG. 1 illustrates the usefulness of HDD by demonstratingthat a borehole 12 can be made without disturbing an above-groundstructure, namely a roadway or walkway as denoted by reference numeral14. To cut or drill the borehole 12, a drill string 16 carrying acutting tool such as a drill bit 18 is rotationally driven by a rotarydrive system 20. When the HDD system 10 is used for drilling a borehole12, monitoring the position of the drill bit 18 is critical to accurateplacement of the borehole and subsequently installed utilities. Thepresent invention is also useful in tracking the progress of a cuttingtool such as a backreamer used to enlarge a borehole. The presentinvention is directed to a system 22 and method for tracking andmonitoring a downhole tool 24 during an HDD operation.

The HDD system 10 of the present invention is suitable fornear-horizontal subsurface placement of utility services, for exampleunder the roadway 14, building, river, or other obstacle.

The tracking system 22 for use with the HDD system 10 is an airborneself-propelled autonomous receiver particularly suited for providing anaccurate three-dimensional locate of the downhole tool assembly 24 fromabove ground. The locating and monitoring operation with the presentreceiver system 22 is advantageous in that it may be accomplished in asingle operation that does not require the operator to stand on theborepath or above the downhole tool. The present invention also permitsthe position of the downhole tool assembly 24 to be monitored withoutrequiring the tracking system 22 be placed directly over a transmitterin the downhole tool assembly. These and other advantages associatedwith the present invention will become apparent from the followingdescription of the preferred embodiments.

With continued reference to FIG. 1, the HDD system 10 comprises thedrill rig 28 having a rotary drive system 20 operatively connected tothe first end of the drill string 16. The downhole tool 24 is connectedto the second end of the drill string 16. The downhole tool 24preferably comprises an electronics package 30 o and has a cutting toolsuch as a slant-faced drill bit 18 connected to its downhole end. In apreferred embodiment the transmitter is supported within a housing ofthe downhole tool 24. However, an alternative transmitter as disclosedin co-pending U.S. patent application Ser. No. 14/733,340 may be usedwithout departing from the spirit of the present invention. Theelectronics package 30 comprises a transmitter 32 for emitting a signalthrough the ground. Preferably the transmitter 32 is supported by thedownhole tool 24 and comprises a dipole antenna that emits a dipolemagnetic field. The electronics package 30 may also comprise a pluralityof sensors 34 for detecting operational characteristics of the downholetool 24 and the drill bit 18. The plurality of sensors 34 may generallycomprise sensors such as a roll sensor to sense the roll position of thedrill bit 18, a pitch sensor to sense the pitch of the drill bit, atemperature sensor to sense the temperature in the electronics package30, and a voltage sensor to indicate battery status. The informationdetected by the plurality of sensors 34 is preferably communicated fromthe downhole tool assembly 24 on the signal transmitted by thetransmitter 32 using modulation or other known techniques.

With reference now to FIG. 2, shown therein is a preferred embodiment ofthe tracking system, or receiver 22 of the present invention. Thereceiver 22 comprises a frame 36, a computer processor 38, and anantenna assembly 40 supported by the frame. The processor 38 issupported on the frame 36 and operatively connected to the antennaassembly 40. The frame 36 is preferably of lightweight construction andcapable of being lifted and maneuvered with a propulsion system 42supported by the frame and operatively connected to the processor 38.The propulsion system 42 may comprise one or more rotors 43 used to liftthe frame into the air. The receiver shown in FIG. 2 comprises aquadcopter having a housing 44 for supporting the antenna assembly 40.While a quadcopter is shown to illustrate the usefulness of the presentinvention, one skilled in the art will appreciate any remotelycontrolled or autonomous aircraft capable of lifting, hovering, landing,and moving the antenna assembly will be an acceptable vehicle for movingthe antenna assembly.

The antenna assembly 40 is supported on the frame 36 and is preferablyadapted to measure the total magnetic field emitted by the dipoletransmitter 32. The antenna assembly 40 may comprise three mutuallyorthogonal antennas which measure the magnetic field along theirspecific axis of sensitivity. Each of the three orthogonal antennasignals is squared, summed, and then the square root is taken to obtainthe total field. This calculation assumes the sensitivities of eachantenna are the same and that the center of each antenna is coincidentwith the other two such that the antenna arrangement is measuring thetotal field at a single point in space. As shown in FIG. 2 the tri-axialantenna may comprise three antenna coils wound around a frame inchannels formed in a support structure. The structure and function ofsuch an antenna is described more fully U.S. Pat. No. 7,786,731, issuedto Cole et al., the contents of which are fully incorporated herein.While wound antenna coils are shown in FIG. 2, one skilled in the artwill appreciate that printed circuit board antennas like those disclosedin co-pending U.S. patent application Ser. No. 14/750,553, ortraditional ferrite rod antennas may be used without departing from thespirit of the invention. Additionally, more than one antenna assembly 40may be supported by the frame as disclosed in U.S. patent applicationSer. No. 14/137,379, the contents of which are incorporated fully hereinby this reference.

A processor 38 may be supported on the frame and programmed to determinea distance between the antenna assembly 40 and the transmitter 32(FIG. 1) based on the signal strength and magnetic field orientationmeasurements taken by the antenna assembly. Alternatively, the processor38 may be replaced by a communication system adapted to transmit themeasurements taken by the antenna assembly 40 to a processor disposed ata location remote from the receiver 22 such as at drill rig 28. Such anarrangement may be preferable if the weight of the receiver componentssupported by the frame is of a concern.

An altimeter 46 may be supported by the frame 36 and used to determinean altitude (height above ground level) of the frame. The altimeter 46may comprise a traditional altimeter or ultrasonic or Ultra Wide Band(UWB) sensors. Alternatively, a global positioning system may be used todetermine the position and altitude of the receiver. Knowing thealtitude of the frame 36, and thus the antenna assembly 40, is importantfor determining the depth of the transmitter 32 (FIG. 1) below ground.The depth of the transmitter 32 is determined using the altitude of thereceiver and the distance between the antenna assembly 40 and thetransmitter. In other words, when the antenna assembly 40 is directlyover the transmitter 32 the altitude is subtracted from the distancebetween the receiver 22 and the transmitter to determine a depth of thetransmitter. This function is preferably performed by the processor 38and communicated to a hand-held remote display (not shown) or via aremote display (not shown) at the drill rig 28.

The processor 38 is programmed to transmit a command signal to thepropulsion system 42. The command signal instructs the propulsion system42 and causes the receiver 22 to move to a position above thetransmitter 32. The command signal may direct the frame to move to anull point of the magnetic field above and in front of the transmitter32 in a manner yet to be described.

With reference now to FIG. 3, shown therein is a block diagram of thepreferred embodiment of the receiver 22 of the present invention. Theantenna assembly 40, as described earlier, measures changes in themagnetic field. A change sensed in the magnetic field will result in avoltage being induced in response to the transmitter's 32 magneticfield. The voltages from the antennas 40 x, 40 y, and 40 z are sent tofilter 48 and amplifier 50. Filter 48 eliminates the effects of othersignals received by the antennas 40 from local noise sources. Amplifier50 increases the signal received by the antennas 40 x, 40 y, and 40 z.An A/D converter 52 is used to convert analog waveform information intodigital data.

The digital data from the A/D converter 52 is then sent to the centralprocessor 38 (CPU) to calculate the location of the transmitter 32(FIG. 1) relative to the receiver 22. The CPU 38 may comprise a digitalsignal processor (DSP) and a microcontroller. The CPU 38 decodes theinformation from the A/D converter 52 and performs calculations todetermine the location of the transmitter 32. The CPU 38 may alsodiscern information transmitted on the magnetic field, to determine thebattery status, pitch, roll, and other information about the downholetool assembly 24.

The receiver 22 may comprise one or more sensors 54 used to senseoperational information about the receiver 22. For example, thealtimeter 46 (FIG. 2), one or more accelerometers, or other knowninclination and orientation sensors or magnetic compasses, may provideinformation concerning the roll or tilt of the receiver 22. Anorientation sensor may be used to determine an orientation of the frame36 relative to a reference orientation. Commonly, the referenceorientation would comprise the frame disposed in a level orientationrelative to the horizon. Information from the sensors 54 is provided tothe A/D converter 52 and to the CPU 38 where the DSP may makecalculations to compensate for the receiver 22 not being level.

In the preferred embodiment a user interface 56 having a plurality ofbuttons, joysticks, and other input devices may be used to control thereceiver 22. The operator can input information for use by the CPU 38through the user interface 56. Information entered through the userinterface 56 or determined or used by the CPU 38 may be displayed to theoperator on a visual display (not shown) screen at the receiver 22. Thereceiver 22 also comprises a radio 58 having an antenna 6 o fortransmitting information from the CPU 38 to the remote user interface 56via antenna 62, such as at the drilling machine 10.

The receiver 22 is preferably powered by a battery assembly 64 and powerregulation system 66. The battery assembly 64 may comprise rechargeablebatteries. The power regulation system 66 may comprise a linearregulator or switch mode regulator to provide power to the variouscomponents of the receiver 22.

The processor 38 receives the magnetic field measurements taken byantennas 40 x, 40 y, and 40 z and processes them as disclosed in U.S.Pat. No. 7,786,731 to determine the location of the transmitter 32 oralternatively to direct the receiver 22 to the transmitter. However,instead of translating the inputs into directional indicators used todirect the operator to certain points in the magnetic field, theprocessor 38 issues command signals that direct the propulsion system 42to move the receiver 22 in a desired direction. The processor 38 maydirectly interface with the propulsion system 42 controls or it maytransmit the antenna signals received from the antenna assembly 40 to aremote processor via a wireless communication link adapted to controlmovement and position of the tracking receiver 22. Use of a processorremote from the tracking receiver 22 will reduce the weight of thereceiver and reduce power consumption from the batteries 64.

Referring now to FIG. 4, there is shown therein a graphical depiction offlux lines radiating from the transmitter 32 in the x-z plane. Assumingthe pitch of the receiver 22 is 0, note that the angle α⇒0 as z⇒0. Theprocessor 38 detects this angle and commands the propulsion system 42 tomove the receiver 22 until it is located above a front null point 68 (apoint wherein the magnetic field is completely vertical). Using theantenna assembly 40 the front null point 68 is easily determined bydetecting a signal strength measurement of zero with antenna 40 y. Atthis point, the receiver 22 will be located on the borepath above and infront of the transmitter 32. Field strength measurements may be taken atthe front null point 68 with the 40 x and 40 z antennas to determine thedirect distance to the transmitter 32. Using the direct distance and thedistance between the receiver 22 and the ground, the depth of thetransmitter 32 can be determined.

With the present invention, improved methods for directing and drillinga horizontal directional borehole 12 are also possible. For example, areceiver having an antenna assembly for detecting the dipole magneticfield in three dimensions and a propulsion system to lift the receiveroff the ground is provided. The dipole magnetic field is transmittedfrom the transmitter 32 and the propulsion system is engaged to lift thereceiver 22 into the air. The antenna assembly 40 continuously detectsthe magnetic field. Signal strength and field orientation measurementstaken by the antenna assembly 40 are used by the processor 38 todetermine a location of the receiver 22 within the magnetic field and todirect the receiver to a position above the transmitter 32 that iswithin a cone having a vertex at the transmitter, a vertical axis, andboundaries defined by the front and back null points. At the front nullpoint 68 the processor 38 may take measurements of the signal strengthof the field to determine a location, including the depth, of thetransmitter 32. Measurements may be taken with the receiver 22 hoveringabove the ground. Alternatively, the receiver 22 may land to take fieldstrength measurements and then take-off to move to a new location.Landing to measure the magnetic field may be advantageous to reducenoise effect from the propulsion system 42 when locating the transmitter32 or to steady the receiver 22 if high winds are present.

In an improved method of tracking the downhole tool, the transmitter 32may be moved along the desired borepath and the receiver 22 may beprogrammed to automatically move with the transmitter to maintain itsposition at the front null point 68 and provide periodic depth andlocation measurements as the boring operation advances.

A second receiver 22 b may also be utilized in concert with the receiver22 to track the downhole tool as it progresses along the borepath. Insuch system, the second receiver 22 b may be programmed to find andposition itself at a back null point 70 with the receiver 22 positionedat the front null point 68. Onboard sensors or GPS may be used todetermine the direct distance between the receiver 22 and secondreceiver 22 b. With the receivers 22, 22 b positioned at the null points68 and 70 the processer may determine the depth of the transmitter 32which is equal to the distance between the receivers divided by thesquare root of 2 (assuming the pitch of both is zero).

In another embodiment, as illustrated in FIGS. 8-9, an absoluteelevation (that is, the height above sea level) may be selected by anoperator for the receiver 22. The altimeter 46 may include GPS locationallowing the receiver's elevation above sea level, as well as localaltitude above ground level, to be monitored. Additional sensors maydetect obstructions 201, such as trees and power lines as shown in FIG.8, which extend above ground level. As used herein, altitude refers tothe distance above ground 202 level, while elevation refers to distanceabove the sea level of the Earth.

As shown in FIG. 9, a planned borepath 203 may extend under unevenground 202 along a substantially constant grade. Such substantiallyconstant grades are advantageous when planning utilities in which flowis gravity driven, such as sewer or stormwater utilities.

In order to plan the borepath 203, aerial tracker 22 may be programmedto conduct a pre-flight of the region to be bored. During the pre-flightthe aerial receiver 22 may utilize onboard sensors 46 to detect andrecord ground 202 elevation as well as above ground obstacles 201 thatmay obstruct the aerial receiver along the borepath 203. Based onrecorded elevation, altitude above the ground 202 and obstacles 201, theprocessor 38 may be configured to automatically pick the optimalelevation based on the lowest clear path. Alternatively, the operatormay select the elevation on a user interface displaying the pre-flightresults.

From the selected elevation the aerial receiver 22 will fly in ahorizontal plane above the planned bore 12. The elevation of the aerialreceiver 22 remains unchanged as it flies over varying terrain, as shownin FIG. 9. As ground elevation changes the altitude of the receiver 22above the ground vary along the borepath 203. The selected elevation canbe maintained by the aerial receiver 22 using an altimeter, GPS system,or other known methods in the art.

Since the aerial receiver 22 maintains a constant elevation, the depthof the transmitter 32, and thus the downhole tool 24 can be determinedin relation to a constant reference plane 220. As shown, the referenceplane 220 is a horizontal plane at a constant elevation. By using ahorizontal plane for depth below the receiver 22, the slope between twounderground waypoints 210 can be calculated without the need forconsidering the differences in ground elevation between the above groundreference points 211 directly above the waypoints 210. Alternatively,the reference plane 220 may have a slope between waypoints 210 whichmatches the desired slope of the borepath. In such a configuration,separate reference planes 220 may be utilized between each set ofwaypoints 210.

The underground depth 222 of the transmitter 32 may also be measured andrecorded by the aerial receiver 22. The measured depth 222 is thedetected vertical distance between the horizontal plane 220 and thebeacon less the altitude 223, which is the distance between the aerialreceiver 22 and the ground 202 level. Distance between the aerialreceiver 22 and ground 202 level may be measured with a radar altimeteror other similar method.

Additionally, the aerial receiver 22 may be maintained at a constantvertical distance above the transmitter 32. Depth 222 may be determinedin such a configuration by subtracting the receiver 22 altitude 223 fromthe vertical distance.

Alternatively, the aerial receiver 22 may be maintained at a constantvertical distance above the planned position of the transmitter 32according to the borepath plan. Deviations from depth detected by thereceiver 22 may be corrected through steering instructions.

It may be preferable to vary the elevation of the receiver 22 during thebore operation. For example, it may be necessary to increase elevationto re-establish radio communication with the HDD machine 20 or forobstacle 201 avoidance. Onboard sensors may detect either stationary ormoving obstacles and require avoidance maneuvers. It may also bebeneficial to lower elevation to increase or verify accuracy of theaerial receiver 22 or to periodically land the aerial receiver 22 toincrease battery life. In any case, the aerial receiver 22 may return tothe selected elevation to record depth from the selected referenceframe.

With reference now to FIG. 5, two alternative locating receivers 100 areshown. The receivers 100 are used above a surface of the ground 102 tolocate an underground line or utility 104. The underground line 104 hasa length and generates a magnetic field 106. The magnetic field 106 maybe placed on the line directly as in U.S. Pat. No. 5,264,795, issued toRider, et al., the contents of which are incorporated herein byreference.

As shown in FIG. 7, the underground line 104 extends from a first end140 to a second end 142. A signal generator 144 places a signalfrequency directly on the underground line 104 at its first end 140.Alternatively, the generator 144 may induce the signal on theunderground line 104.

With reference again to FIG. 5, the field 106 generated around theunderground line 104 is “cylindrical”. This does not mean that the field106 itself is a cylinder, but that it approximates a cylinder as itfollows the often-curved length of the underground line 104. Therefore,the receivers 100 are located directly over the underground line 104when the x component of the field 106, as shown in FIG. 5, is zero. Thereceivers 100 may have onboard sensors or GPS locators to determine thedirect distance between them.

Depth of the underground line 104 below the ground 102 is determined bysensing the strength and/or shape of the magnetic field 106 and theheight of the receiver 100 over ground level. It may be advantageous toperiodically land the receivers 100 on the ground 102 to calibrate thealtitude of the receivers above ground level.

With reference now to FIG. 6, the receiver 100 is shown in more detail.The receiver 100 comprises a first antenna assembly 110 and a secondantenna assembly 112. The first antenna assembly 110 is verticallydisplaced from the second antenna assembly 112. Various antennas may beused, including ferrite core, air core, and the antenna assemblydescribed with reference to FIG. 3.

Continuing with FIG. 6, the receiver 100 further comprises a frame 114and propulsion system 116 similar to that found on the receiver 22 ofFIG. 2. Preferably, four rotors 118, such as those used in a“quad-copter” design are used to propel the frame 114. A pair of legs120 are slightly further from the frame 114 than the second antennaassembly 112 such that when the receiver 100 is placed on the ground,the second antenna assembly is just off the ground.

Each antenna assembly 110, 112 is adapted to detect the total magneticfield emanating from the underground line 104 (FIG. 5). A processor 38may be supported on the frame 114. The first 110 and second 112 antennaassemblies are a known distance apart, and make measurements based uponsignal strength and magnetic field orientation measurements.Alternatively, the processor 38 may be remote from the frame 114. Ineither case, the antenna assemblies 110, 112 send an antenna signal tothe processor 38, which uses the signal to generate a command signal.

Sensors 122, such as an altimeter, global positioning system (GPS)receiver, or other known devices, may determine the elevation of areference point on the receiver 100 over the ground 102 (FIG. 5). Thiselevation may be subtracted from the distance between the referencepoint and the underground line, as measured by the antenna assemblies110, 112.

A command signal may be generated by the processor 38 to cause thereceiver 100 to move along the underground line 104, as indicated by theshape of the magnetic field. In this way, the path and depth of theunderground line 104 may be mapped, even when the terrain directly abovethe line does not permit use of an on-ground locator.

Various modifications can be made in the design and operation of thepresent invention without departing from its spirit. Thus, while theprinciple preferred construction and modes of operation of the inventionhave been explained in what is now considered to represent its bestembodiments, it should be understood that within the scope of theappended claims, the invention may be practiced otherwise than asspecifically illustrated and described.

1. A system for tracking a drill bit, the system comprising: a drillrig; a drill string having a first end and a second end, the first endis operatively connected to the drill rig; a downhole tool connected tothe second end of the drill string; a dipole magnetic field transmittersupported by the downhole tool, in which a dipole magnetic field isemitted from the downhole tool at an underground location; a drill bitconnected to the downhole tool; and a self-propelled autonomous receivercomprising: an antenna to detect the dipole magnetic field; and aprocessor, in which the processor is configured to perform a methodcomprising: maintaining the autonomous receiver in a reference planeabove the ground; receiving a signal indicative of the field detected bythe antenna; determining a direction of a null within the dipolemagnetic field; and directing the autonomous receiver to move along thereference plane to the null point.
 2. The system of claim 1, wherein theprocessor performs a method further comprising: upon the receiverreaching the null point, receiving a signal indicative of the fielddetected by the antenna; and determining a vertical distance from thereference plane to the dipole magnetic field transmitter using thereceived signal.
 3. The system of claim 1 in which the autonomousreceiver further comprises: a positional sensor to detect the absoluteposition of the receiver in three dimensions; and a propulsion systemconfigured to autonomously move the null field location in response to acommand signal from the processor.
 4. The system of claim 3 wherein theautonomous receiver further comprises: an altimeter.
 5. The system ofclaim 3 in which the propulsion system comprises at least one helicopterrotor.
 6. The system of claim 3 wherein the method performed by theprocessor further comprises: detecting the altitude of the autonomousreceiver and determining the actual depth of the transmitter below theground using the altitude and the vertical distance.
 7. The system ofclaim 1 in which the reference plane is at a constant elevation.
 8. Amethod of using the system of claim 1, in which the underground locationis characterized as a first underground location, the method comprising:after determining the vertical distance from the reference plane to thedipole magnetic field transmitter using the signal received from thefirst underground location: advancing the drill string and the downholetool to a second underground location; emitting a dipole magnetic fieldfrom the dipole magnetic field transmitter at the first undergroundlocation; moving the autonomous receiver to a null point of the magneticfield with the downhole tool at the second underground location; andthereafter, determining a vertical distance from the reference plane tothe dipole magnetic field transmitter at the second underground locationusing the received signal.
 9. The method of claim 8 further comprising:determining a slope of a reference line, the reference line extendingfrom the first underground location to the second underground location,using the detected vertical distances at the first underground locationand the second underground location.
 10. The method of claim 8 furthercomprising the steps of: providing a borepath plan comprising planneddepths of the downhole tool; determining the vertical distance betweenthe reference plane and the dipole magnetic field transmitter;simultaneously, detecting an altitude of the autonomous receiver todetermine an actual depth from the altitude of the receiver above theground and the vertical distance; and comparing the actual depth withthe borepath plan to generate a depth error value.
 11. The method ofclaim 10 further comprising steering the drill string to providecorrections to the depth of the downhole tool in response to thegenerated depth error value.
 12. A method comprising: transmitting adipole magnetic field from a transmitter at an underground location;engaging a propulsion system to lift an autonomous receiver into the airto a predetermined reference elevation; detecting the dipole magneticfield at the reference elevation using an antenna assembly disposed onthe autonomous receiver; moving the receiver with the propulsion systemto a position above the transmitter and at a front null point of themagnetic field using the detected magnetic field while keeping thereceiver at the predetermined reference elevation; thereafter, measuringthe signal strength of the magnetic field and the orientation of themagnetic field in three dimensions using the antenna assembly with thereceiver at the null point to determine a vertical distance between thetransmitter and the receiver; detecting an altitude of the receiverabove a ground surface; and thereafter, determining an actual depth ofthe underground location using the altitude and the vertical distance.13. The method of claim 12 in which the autonomous receiver ischaracterized as a first receiver, and further comprising: engaging apropulsion system to lift a second receiver in the air to apredetermined reference elevation; detecting the dipole magnetic fieldat the reference elevation using an antenna assembly disposed on thesecond receiver; moving the second receiver to a position above thetransmitter and at a rear null point of the magnetic field using thedetected magnetic field while keeping the receiver at the predeterminedreference elevation.
 14. The method of claim 13 further comprising:determining a distance between the receiver and the second receiver; anddetermining a depth of the transmitter below the reference elevation onthe distance between the receiver and the second receiver; moving thetransmitter; and automatically moving the first receiver with the frontnull point and the second receiver with the back null point as thetransmitter is moved.
 15. The method of claim 12 in which thetransmitter is disposed at a downhole tool located at a distal end of adrill string.
 16. A method, comprising: providing a borepath plancomprising planned depths of the downhole tool at a series ofunderground waypoints; determining the vertical distance between thereference elevation and the transmitter at at least one waypoint;simultaneously, detecting an altitude of the autonomous receiver todetermine an actual depth using the altitude of the receiver above theground and the vertical distance; after the preceding steps, performingthe steps of claim 12; and thereafter, comparing the actual depth withthe planned depth to generate a depth error value.
 17. The method ofclaim 12, further comprising: prior to transmitting the magnetic field:flying the autonomous receiver to above a ground region above the siteof a planned below-ground borepath; detecting obstacles above the groundregion with the autonomous receiver; and thereafter, choosing thereference elevation based on a position of the planned below-groundborepath as compared to the detected obstacles.
 18. The method of claim12, wherein the step of moving the receiver with the propulsion systemto a position above the transmitter and at a front null point of themagnetic field using the detected magnetic field further comprises:moving the receiver to the null point at the reference elevation. 19.The method of claim 12, wherein the step of moving the receiver with thepropulsion system to a position above the transmitter and at a frontnull point of the magnetic field using the detected magnetic fieldfurther comprises: moving the receiver to the null point below thereference elevation.
 20. A system comprising: a signal transmitterdisposed at an underground location and generating a dipole magneticfield from the underground location; and a self-propelled autonomousreceiver comprising: an antenna assembly to detect the magnetic fieldand generate an antenna signal; a processor programmed to receive theantenna signal and generate a command signal; and a propulsion system toreceive the command signal and move the receiver to a position above thetransmitter and at a front null point of the magnetic field using thedetected magnetic field while keeping the receiver at a predeterminedreference elevation.